The invention relates to electret transducers and their manufacture.
Electret transducer microphones include an electret between a pair of spaced, plate-shaped electrodes, one electrode being flexible so that sound waves cause changes in spacing and the electrical characteristics between the two. Electret transducer microphones used in hearing aids typically act as parallel plate capacitors and have a rigid plate electrode that carries an electret for bias and a spaced flexible diaphragm plate electrode. Typically, the rigid plate electrode is directly coupled to a field effect transistor (FET) preamplifier. As hearing aid microphones become smaller, their capacitance becomes smaller, and the effect of stray capacitance on signal loss becomes pronounced. Usually the fixed (i.e., rigid) electrode is connected to the FET gate, and the flexible diaphragm electrode is connected to a ground which is connected to the FET source. To reduce stray capacitance, the gate-connected electrode should be well spaced from ground and grounded supporting members; providing such spacing becomes more difficult as the size of microphones becomes smaller.
Stray capacitance is a problem for both conventional microphones with metal fixed electrodes, often referred to as backplates, and for more recent designs including silicon (which has adequate conductivity to be considered the equivalent of metal) backplates. Examples of the more recent silicon designs are described in Hohm, D. and GerhardMulthaupt, R., "Silicon Dioxide Electret Transducer", J. Acoust. Soc. Am., Vol. 75, pp. 1297-1298 (April 1984), and Sprenkeis, A. and Bergveld, P., "Development of a Subminiature Electret Microphone Constructed in Silicon", Proc. 4th Conf. Solid State Sensors and Aotuators, IEEE, Tokyo, 1987, p. 295.